1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device which prevents a defective display by suppressing the generation of gas from thin films made of various organic materials which are formed on an inner surface of a substrate, and a fabrication method of the liquid crystal display device.
2. Description of the Related Art
As display devices for personal computers, word processors and other information equipment, recently, thin and light-weighted display devices of low power consumption which use liquid crystal display elements have been popularly used. The liquid crystal display element is basically configured such that a matrix is formed of a large number of electrodes which are arranged horizontally and vertically, a liquid crystal layer is formed between the above-mentioned horizontal and vertical electrodes, and each pixel is formed at a portion where two electrodes face each other in an opposed manner. Due to such a constitution, the liquid crystal display element can display a two-dimensional image.
This type of liquid crystal display device is classified into a so-called single matrix type liquid crystal display device in which given pixels are selected at timings of pulses applied to the horizontal and vertical electrodes and a so-called active matrix type liquid crystal display device in which a non-linear type element such as a transistor is arranged for every pixel which is formed at a crossing point of the vertical and horizontal electrodes and given pixels are selected.
For example, in a liquid crystal display device which uses active-matrix type liquid crystal display elements, non-linear elements (switching elements) are formed corresponding to a plurality of respective pixel electrodes arranged in a matrix array. Liquid crystal in each pixel is theoretically always driven (duty ratio: 1.0) and hence, compared to the so-called single matrix type which adopts a time-division driving method, the active matrix type shows a more favorable contrast. Accordingly, the active matrix type has been considered as an indispensable technique in the color liquid crystal display device. As a typical example of the switching element, a thin film transistor (TFT) is named.
The active matrix type liquid crystal display device which uses the thin film transistors is known through, for example, JP-A-63-309921 or xe2x80x9c12.5 type active matrix type color liquid crystal display adopting a redundancy constitutionxe2x80x9d (Nikkei Electronics, 193-210 pages, Dec. 15, 1986, published by Nikkei McGraw-Hill Ltd.).
Further, in the color liquid crystal display device of this type, a plurality of color filters (generally red, green and blue) are provided to one substrate (color filter substrate, hereinafter referred to as xe2x80x9ccounter substratexe2x80x9d) out of a pair of substrates which sandwich a liquid crystal layer. In general, the color filters are formed by applying organic materials in which organic colored material such as dyes or pigments are mixed and patterning is repeated for respective three color filters using a photolithography method. Here, a light shielding film, that is, a black matrix which separates respective colors is formed by a similar technique before or after forming the color filters.
Further, to level an inner surface of a color filter substrate for making a gap defined between opposing faces of a pair of substrates, that is, a cell gap, uniform, a leveling layer (protective film: overcoat layer) made of organic material as disclosed in JP-A-5-140267, for example, is formed on color filters so as to cover the color filters.
In the liquid crystal display device which is constituted by laminating the color filter substrate on which such a thin film made of organic material or the like is formed to another substrate (active matrix substrate, hereinafter referred to as xe2x80x9celectrode substratexe2x80x9d), there has been a drawback that bubbles are generated when an impact is applied to the liquid crystal display device.
To cope with such a drawback, conventionally, as disclosed in JP-A-8-234188, a method which bakes a so-called empty cell before laminating a color filter substrate and an active matrix substrate to each other and filling liquid crystal between the substrates using an absorption gas removing processing which is mainly represented by hydrated water has been adopted without any concrete aims.
Further, JP-A-2000-171785 discloses an invention in which by using a thin organic material film which exhibits an integrated quantity of detected ion intensity of carbon dioxide measured from room temperature to 250 degree centigrade (Celsius: xc2x0 C.) of equal to or less than 70000 when a liquid crystal display device is subjected to temperature elevation using a heating and gas generating device, the generation of bubbles when an impact is applied to a display screen of the liquid crystal display device can be suppressed.
Usually, in a liquid crystal display device which uses a counter substrate on which color filters are formed, a major portion of a substrate constituting element of the counter substrate is, different from an electrode substrate, made of organic material. Accordingly, the counter substrate per se has a gas generating potential or generates a decomposing gas by a damage due to an external factor and hence, it has been confirmed that when the liquid crystal display device is stored or used at a normal temperature and a normal pressure, gas is dissolved in the liquid crystal.
When the gas which is dissolved in the liquid crystal in this manner, due to the characteristics of the liquid crystal display device which is constituted of a substrate having high flexibility such as the counter substrate formed of a thin glass substrate which is generally liable to be easily deformed due to an external force, when a display screen is pushed with a finger or an impact is applied to the display screen due to a fall of the liquid crystal display device, bubbles are generated in the liquid crystal display device and this may cause a defective display.
Further, when the gas is dissolved into the inside of the liquid crystal, the characteristics of the liquid crystal display per se, for example, a resistivity of the liquid crystal is degraded and this may cause a defective display.
Accordingly, it is an object of the present invention to provide a liquid crystal display device which ensures high reliability by preventing the generation of bubbles from various thin formed films, particularly from organic material films such as a leveling layer even when the liquid crystal display device is stored for a long time or even when an external force is applied to the liquid crystal display device during use thereof.
To achieve the above-mentioned object, the present invention is characterized in that in a liquid crystal display device which arranges a pair of substrates having organic thin films such as leveling layers and forming orientation films for orienting liquid crystal molecules constituting liquid crystal on uppermost layers of inner surfaces thereof to face each other in an opposed manner with a fixed gap and adheres both peripheries of the substrates with a sealing material and fills liquid crystal in the gap hermetically, the characteristics of the organic thin film and the forming temperature of a thin film of an oxide conductive film are specified.
One typical example of the liquid crystal display device according to the present invention is described as following structures (1) to (5).
Structure (1):
In a liquid crystal display device which is configured such that the display device includes a substrate which has a thin organic material film formed on an inner surface thereof and a thin conductive oxide film formed to cover at least a part of the thin organic material film, liquid crystal is hermetically filled between the substrate and another substrate which faces the substrate in an opposed manner, and both substrates have peripheries thereof adhered to each other by a seal material, a thermal decomposition temperature of the thin organic material film is not lower than a temperature for forming the thin conductive oxide film, and a heat deflection temperature of the thin organic material film is not higher than the temperature for forming the thin conductive oxide film.
Structure (2):
In the above-mentioned structure (1), the thermal decomposition temperature of the thin organic material film is not lower than 220 degree centigrade (xc2x0 C.).
Structure (3):
In the above-mentioned structure (1) or (2), the heat deflection temperature of the thin organic material film is not lower than 200 degree centigrade (xc2x0 C.).
Structure (4):
In any one of the above-mentioned structures (1) to (3), the organic material is formed of epoxy resin.
Structure (5):
In any one of the above-mentioned structures (1) to (4), retention ratio (retentivity, hereinafter) of resistivity of the liquid crystal is equal to or larger than 10% of resistivity of the liquid crystal in an unused condition (that is, as virgin liquid crystal).
Further, a typical example of a fabrication method of a liquid crystal display device according to the present invention is described as following processes (6) to (8).
Process (6):
In a fabrication method of a liquid crystal display device which is configured such that the display device includes a substrate which has a thin organic material film formed on an inner surface thereof and a thin conductive oxide film formed to cover at least a part of the thin organic material film, liquid crystal is hermetically filled between the substrate and another substrate which faces the substrate in an opposed manner, and both substrates have peripheries thereof adhered to each other by a seal material, the method comprises:
a step for forming the thin conductive oxide film at a temperature which falls in a range neither higher than a thermal decomposition temperature of the thin organic material film nor lower than a heat deflection temperature of the thin organic material film.
Process (7):
In the above-mentioned process (6), the thin organic material film is formed at a temperature which falls in a range between 200 degree centigrade (xc2x0 C.) and 220 degree centigrade (xc2x0 C.).
Process (8):
In the above-mentioned process (6) or (7), epoxy resin is used as a material of the thin organic material film.
With the use of the above-mentioned substrate, the generation of bubbles from the organic material film can be prevented. Further, it is also possible to suppress the generation of gas (bubbles) in the liquid crystal display device during storage or during use. Accordingly, the liquid crystal display device of high quality and high reliability can be obtained.
Further, by performing the formation of the above-mentioned thin oxide conductive film at a temperature which is equal to or lower than the thermal decomposition temperature of the thin organic material film and equal to or more than the heat deflection temperature of the thin organic material film, the generation of gas (bubbles) in the liquid crystal display device during storage or during use can be suppressed whereby the liquid crystal display device of high quality and high reliability can be fabricated.
The present invention is not limited to the above-mentioned constitutions and various modifications are conceivable within a scope of the technical concept of the present invention.